The present invention pertains broadly to airhouses for supplying conditioned air to the interior of an enclosed building, and more particularly to an improved airhouse compatible with existing systems and having reduced initial cost and increased operating efficiency.
In a system developed in recent years by Applicant for maintaining desired atmospheric conditions throughout large industrial-type buildings, the building interior is substantially isolated or enclosed and conditioned air, including sufficient outside air, is continuously admitted through air handling units, or airhouses, located about the building to maintain the interior of the building under a slight positive pressure throughout, slightly greater than the effective outside atmospheric pressure around the building. Various aspects of such a system are described, by way of example, in U.S. Pat. Nos. 4,850,264; 4,960,041; and 5,146,977, as well as the above-identified U.S. patent applications. As discussed at length therein, sensors strategically located throughout the building monitor various atmospheric conditions such as relative interior and exterior atmospheric pressures, temperature, humidity, pollutants and particulate levels, and transmit signals indicative of the readings to a central computer referred to by Applicant as a global control computer. The computer periodically monitors the signals and compares them to stored data representing desired parameters for the atmospheric conditions in the various zones monitored by the sensors. The controllable airhouses strategically located about the building, generally on the roof, are individually regulated in response to signals from the central computer to collectively take corrective measures when a trend is detected for an atmospheric condition in a particular area to fall outside the predetermined parameter.
Airhouses used heretofore in such systems have generally included a sheet metal cabinet housing a heat source, typically a direct-fired gas burner or steam coil, a fan or blower unit, and a system of dampers for regulating admission of air to and discharge of air from the airhouse. The airhouse might also include evaporative humidification equipment. Such airhouses perform well for their intended purpose. However, they have certain limitations which the present invention overcomes.
The airhouses are generally installed on the roof of the structure, and installation of each unit requires substantial modification to the roof structure, so that multiplication of the units adds to the overall cost. In addition, the prior art units have a fixed capacity, that is, the blower units operate at a constant rotational speed to output a constant volumetric flow rate of air. The proportion of outside and building return air is modulated by operating dampers in the outside air and building return air inlets in opposition. In order to provide a straight line variation in damper flow capacity, so that the combined flow of the dampers remains constant, the prior art dampers require a significant area in the airhouse.
In large industrial buildings where a number of airhouses are provided there is a need to individually control airhouses to provide desired building pressurization and the intended environmental conditions at various points within the facility. Consequently the global control central computer which monitors various inputs individually modulates the plurality of airhouses. Currently, these airhouses as well as other designs using a single blower operate in a constant speed mode; namely, that during operation, the blower or blowers turn at a fixed speed and thus a constant volumetric flow rate of air is delivered. In some applications there is a desire to modulate blower output which can be accomplished by providing such airhouses which variable blower speed capability, enabling the blowers to operate either at a number of fixed speed settings, or be continuously variable through a range of speed or output settings. One type of continuously variable blower speed system is termed a variable frequency drive (VFD) which uses a variable frequency AC power supply with a motor operating at a synchronous speed. Variable output capabilities would permit greater modulation of building pressure and air temperature control and reduces the need for frequent cycling of airhouse units. It would be preferred to allow units to operate at lower speed settings where building pressurization and air temperature conditions would permit, which would monitor desired building air circulation.
Although variable speed blowers in environmental conditioning units are known per se, there are particular difficulties which must be addressed in operating new generation airhouses as designed by Applicant in this manner. In many installations these new airhouses use direct fire burners to provide heat input to outside air being delivered through the airhouse. Since during operation of direct fire burners, products of combustion are admitted to the building air, careful control over the combustion process is essential. One of the variables which must be controlled is air velocity through the burner. Without taking particular measures to provide constant burner air flow velocity, changes in blower speed settings would also change burner air flow velocity, producing undesirable consequences. Accordingly, there is a need to provide control systems and strategies which provide constant air flow velocity through direct fire burners in the use of Applicant's airhouses.
The use of Applicant's airhouses in large building environmental control systems represents a significant change from prior art approaches. Prior approaches typically involve a number of individually located exhaust fans coupled with distributed heating units having long duct work intended to deliver air to the lower strata of the factory workspace where workers are typically present. These systems are, by comparison, inefficient due to losses attributed to forcing air to flow through long duct work, and typically cause a significant temperature stratification of air within the building. Applicant's approach however uses a number of roof mounted units without substantial duct work which are intended to direct air through the upper levels of the building interior to the lower levels. In order to accomplish this task, certain diffuser exit velocity parameters must be achieved in order to deliver air to the needed locations within the building interior. As mentioned previously, there are situations where variable speed operation is desired. Typical airhouses have a fixed configuration diffuser for directing air flow in a particular manner. Based on fundamental fluid mechanics principles, using a fixed configuration diffuser, exit air velocity would decrease as blower output also decreases. This would have the undesirable consequence of preventing air delivered by the airhouse to reaching its intended target areas within the building interior. Accordingly, there is a need to provide a mechanism for maintaining desired diffuser exit air velocity through a range of blower volumetric output settings.
In accordance with the present invention, there is provided an improved airhouse having greater air handling capacity than conventional airhouses, thereby reducing the number of such airhouses required for a facility. Matched twin blowers are symmetrically arranged within the airhouse and operated in unison to insure a balanced air flow into the airhouse and through the two blowers. In a preferred embodiment, a burner unit mounted intermediate the blowers is flanked by opposed matching outside air inlet dampers. The dampers may be of a roll-up canister construction whereby the open portion of the inlet opening is unobstructed so that the dimensions of the damper opening are minimized. Other damper types such as those described in related U.S. Pat. No. 5,290,188 could also be used. A building return air inlet having adjustable dampers is also provided. The outside air and building return air dampers are operated in opposed fashion so that the combined flow of air entering the airhouse remains at a desired combined flow area. In one embodiment the twin blowers are driven at the same speed by separate motors, while in an alternate embodiment the blowers are mounted upon a common shaft driven by a single motor. Provisions are made for operating the motor or motors at varying speeds to coordinate the output of the airhouse with required air volume under differing operating conditions so as to minimize operating costs. Heat exchangers may typically be provided within the airhouse for heating or cooling the incoming air, and evaporative humidification units may be included for humidifying the air prior to admission to the building interior. An additional outlet may be provided for each blower unit whereby air withdrawn from the building interior may be selectively vented directly to the outside in a so-called "dump mode" of operation.
The airhouses in accordance with the present invention provide a number of features especially adapted for enabling variable blower speed operation. One or more airhouses of a system include a locally positioned microprocessor based control system termed a distributed control system (DCS), which monitors and controls the airhouse at a high response rate. When the centralized global control computer sends a command for a particular unit to change its speed set point, burner air velocity is maintained despite changes in blower setting by adjusting dampers which control the admission of outside air and building return air. These airhouses include sensors for monitoring air velocity through the direct fire burner which provide inputs to the DCS. In prior designs of Applicant's airhouses a reciprocal flow area relationship existed between building return air and outside air inlet area to maintain total flow area constant through changes in quantities of outside air and building return air being admitted. In units with variable speed blower VFD, a reciprocal relationship continues to exist but the total area of inlet-airflow is a function of blower speed setting.
Another feature of the airhouse according to the present invention is the incorporation of variable flow area diffuser assemblies. These units for discharging air from the airhouse have features for changing their cross-sectional flow area in an effort to maintain constant or near constant exit flow velocity through a range of volumetric discharge rates. Control of this feature is automatic in that a static pressure transducer is used in a plenum to which the diffuser is attached. An actuator changes the diffuser flow area in response to inputs from a pressure transducer in the plenum. As blower discharge rate decreases, static pressure in the plenum too will decrease, triggering the diffuser controller unit to actuate the diffuser to decrease flow area, thereby providing greater flow resistance and constant pressure in the plenum. Various configurations of variable discharge diffuser assemblies can be used, including those providing continuously variable flow areas as well as fixed or multiple position devices which provide a step-wise changes in flow resistance. The later devices may be especially useful in connection with multiple speed blowers which do not provide continuously variable speed but rather step-wise output modulation.
This invention further involves control strategies for airhouses incorporating variable blower speed drive systems.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.